| Abstract |
This paper investigates the expected performance of closed-loop geothermal systems. In the past, it has been argued that the rate of thermal conduction through rock masses is not large enough to compensate for the thermal depletion around closed-loop geothermal wells, and that such systems, therefore, cannot maintain the heat extraction rate required for long-term production. Recently, however, because of new technological advances, there is a renewed interest in closed-loop geothermal designs that can exploit the vast energy resources of hot formations unsuitable for conventional hydrothermal geothermal systems. Similar to Enhanced Geothermal Systems (EGS), closed-loop geothermal wells can harvest heat from low permeable and low water content formations; however, in contrast to EGS, closed-loop systems are expected to be more controllable and predictable. They do not involve uncertainties in the nature of fluid flow through the artificially or naturally fractured reservoir, the potential for loss of injected fluid into the surrounding rock mass, nor the potential environmental concerns about induced seismicity, pollution of sub-surface groundwater, and extraction of dangerous elements to the surface. Various designs have been proposed for closed-loop geothermal systems, including co-axial, U-Shape, and multiple string wells. Geothermic Solution LLC (GSL) proposes to use a co-axial closed-loop system with water as the working fluid. The design involves a well with vertical and horizontal sections. The cold water will be injected through the annulus within the well with the outside surface in contact with rock. The heated water will travel to the surface through the inner tube of the well. In this paper, the performance of the GSL well is investigated for a period of 30 years using a numerical modeling approach carried out using FLAC3D, a commercial software developed by Itasca (2015). The model considers heat transfer by (a) conduction through the surrounding rock, (b) advection by fluid circulating in the well, and (c) convection (i.e., heat exchange between the surface of the pipe and the moving fluid). The result of this study is presented along with another closed-loop design, ECO2G technology (GreenFire, 2016; Oldenburg et al., 2016; Higgins et al., 2016), which circulates supercritical carbon dioxide (SCO2) instead of water. ECO2G technology is based on a closed-loop design with a U-Shape well geometry that consists of a vertical injection well, a horizontal section, and a vertical production well. It has been argued that due to its unique properties, SCO2 harvests substantially more energy than water. A series of sensitivity studies with respect to injection temperature and flow rate are carried out for the GSL design with water as the working fluid. The results in each case are compared to results presented for the ECO2 technology under identical conditions (Oldenburg et al., 2016). The generated thermal power for both closed-loop geothermal designs are also presented. The results of this study will show that closed loop systems are very viable and that water is an excellent working fluid to extract heat and that its performance in terms of generated thermal power will be comparable to that for SCO2. However, it is noted that SCO2, while more complex in risk control than water, may have some advantages due to development under certain conditions of thermosiphon. Principally, the cost for sub-surface implementation of both designs are comparable. It is noted that a thorough investigation of economic feasibility requires taking into account the surface facilities converting the thermal energy into end-use energy. This step is beyond the scope of the current paper. Also, the operation costs and procedural aspects, including (a) cost for set-up of the system, or (b) pressure requirements at the inlet and outlet for maintaining the same flow rate in different systems, will not be compared. The results of this study suggest that the range of generated power for different closed-loop systems is close and comparable. One major advantage of these systems is their predictability, i.e., the variability in the expected range of produced power would be significantly lower than the variability expected in EGS system. Therefore, closed loop could be a promising contributor to future geothermal power production. |